The present invention relates to a process for the treatment of radioactive graphite and, in particular, the radioactive graphite which results from the graphite which is used as a moderator material in a number of nuclear designs.
Graphite, which consists predominantly of the element carbon, is used as a moderator in a number of nuclear reactor designs, such as the MAGNOX and AGR gas cooled reactors in the United Kingdom, and the RBMK design in Russia. During construction, the moderator of the reactor is usually installed as an interlocking structure of graphite bricks. At the end of reactor life, the graphite moderator, typically weighing about 2,000 tons, is a form of radioactive waste which requires safe disposal.
Graphite is a relatively stable chemical form of carbon, which is in many ways suitable for direct disposal without processing. However, after neutron irradiation, the graphite will contain stored Wigner energy. The potential for release of this energy needs to be accommodated in any strategy which relies on disposing of the graphite in unprocessed form. Alternatively, processing the graphite before disposal can allow the safe release of any stored Wigner energy.
The graphite also contains significant quantities of radionuclides from neutron induced reactions, both in the graphite itself and in the minor impurities which it contains. The radioisotope content can conveniently be divided into two categoriesxe2x80x94short-lived isotopes and long-lived isotopes. Short-lived isotopes (such as cobalt-60) make the graphite difficult to handle immediately after reactor shutdown, but they decay after a few tens of years. Long-lived isotopes (principally carbon-14) are of concern through the possibility of their discharge to the biosphere. Processing the graphite offers the opportunity to separate the majority of the graphite mass (carbon) from the short-lived radioisotopes. This in turn facilitates disposal of the graphite waste shortly after the end of reactor life, and may permit recycling.
Because of the characteristics of graphite and its mass, the most common procedure to date for decommissioning of graphite moderated reactors is to store the reactor core in-situ for a period of tens of years following reactor shut-down. During this period, short-lived radioisotopes decay sufficiently to allow eventual manual dismantling of the graphite moderator. Most plans in the United Kingdom then assume that the graphite will be disposed of in its existing chemical form, with appropriate additional packaging to prevent degradation or release over the long period of carbon-14 decay.
Storage has certain negative consequences, such as the following: 1) an implication of long-term financial liability, 2) a visually intrusive storage structure that has no productive purpose, and 3) a requirement for a future generation (which gained no benefit from the original asset) to complete eventual clearance. If the storage alternative is to be replaced by shorter term management, it is essential for the graphite to be processed in a safe and radiologically acceptable manner.
Thus there remains a need for a better way to handle radioactively contaminated graphite than storing it.
Accordingly, a preferred embodiment of the present invention provides a process for the treatment and recycling of radioactive graphite that includes the following steps: (i) reacting the radioactive graphite at a temperature within the range of 250xc2x0 to 900xc2x0 C. with superheated steam or gases containing water vapor to form hydrogen and carbon monoxide; (ii) reacting the hydrogen and carbon monoxide from step (i) to form water and carbon dioxide; and (iii) reacting the carbon dioxide from step (ii) with metal oxides to form carbonate salts.
A major feature of the present invention is that the process can be used to separate efficiently the carbon in the graphite from other radioactive elements present in the moderator to allow the ease of handling.
Another major feature of the present invention is that the process can be used to process individual blocks or particles of graphite removed from the reactor core in a conventional manner.
Yet another major feature of the present invention is that the process can also be used to react the moderator graphite slowly, in-situ in a slow and controlled manner without the need for human intervention.
Other features and advantages will become apparent to those skilled in the art from a careful reading of the Detailed Description of Preferred Embodiment accompanied by the following drawings.